As used herein the words "power devices" or "power semiconductor devices are intended to refer to devices carrying currents of about 1 amps or more, and blocking voltages of about 40 volts or more, and the words "chip" or "die" are used interchangeably to describe portions of semiconductor or other electronic wafers or substrates containing one or more electronic elements, as for example but not limited to, transistors, thyristors, diodes, capacitors, resistors, and so forth.
Power devices, especially power semiconductor devices, are widely used in the electronic arts. Such devices are typically enclosed in single device packages, that is, one Power device per package. This is because most power device chips are comParatively large and because there is a great Premium on heat removal from the chip during operation. In order to have efficient heat removal it is generally necessary to have large die flags or heat spreaders.
A great many different arrangements well known in the art are used for packaging individual power devices to suit particular circumstances. For example, the designations TO-3, TO-218, TO-220 are familiar to most electronic designers as representing standardized power device package configurations. Many other standardized package configurations are well known in the art. The dimensions, lead spacings and shape of a particular device package configuration is referred to in the art as the device "footprint" and is used in laying out the lead attachment points and the clearance space on the PC board that will be occupied by the device.
Some of these standardized package configurations are registered with the Joint Electron Device Engineering Council (JEDEC) which maintains a file of agreed body and lead dimensions, arrangements and spacing for the registered package designs. Many different manufacturers offer a wide variety of power devices having different electrical functions and characteristics in these standardized package designs. This simplifies design of the printed circuit (PC) boards to which the devices are intended to attach.
While it has been the habit for many years to package power devices individually, i.e., one power device chip or die per package, there is an increasing need for multi-device packages, that is, unitary assemblies or combinations of several power device chips or combinations of power device chips and integrated circuit (IC) in a single housing or package. This need is especially acute in the automotive and appliance electronics field where driver and/or control circuits frequently require from two to eight or more power devices to run a variable speed motor or drive fuel injectors or ignition systems or anti-skid brake actuators or control some other high current load.
A feature of these circuits is that the multiple power devices must be electrically isolated, that is, not have a common electrical reference connection. This is difficult to accomplish with power devices which dissipate large amounts of heat and where the back side of the semiconductor chip, which is the face through which heat is normally extracted, forms one of the electrical terminals.
Much effort has gone into developing electrically isolated multi-chip packages or assemblies. One approach is to provide a thermally conductive but electrically insulating material (i.e., an isolator) between the power device and the heat sink. Alumina, beryllia and mica are well known isolators. The isolator may be between the semiconductor die and the die flag (i.e., internal to the package), or between the package base and the external heat sink or radiator (i.e., external to the package). The former method adds substantially to the cost of constructing the packaged power device and the latter adds substantially to the cost of assembling the packaged device on the circuit board.
Another approach is to mount multiple devices on a metallized plastic or ceramic insulating substrate. For example, semiconductor die are mounted directly on metallized areas of a miniature PC board with interconnecting wiring thereon to connect the die to external leads. The assembly is then enclosed in some type of encapsulation. This type of construction is comparatively expensive and efficient heat removal is difficult or impossible.
Another approach has been to build several power devices on a single semiconductor die. However, such die become very large, manufacturing yield is low, and this approach is prohibitively expensive for many applications. Accordingly, there continues to be a need for low cost package designs and methods which provide multiple, electrically isolated power devices within a single enclosure.